1. Field of the Invention
This invention relates to an electrophotograhic photosensitive medium useful in electrophotographic processes. More particularly, it relates to a photoconductive medium with an extremely high sensitivity which is usable in any existing electrophotographic processes and which has a wide spectral sensitivity over the wavelength range of visible light.
2. Description of the Prior Art
As is well known, amorphous selenium or selenium alloys which have very excellent electrophotographic characteristics have been widely employed as electrophotographic photosensitive media. In practical applications, selenium or selenium alloys are generally used in the form of a thin film. However, the use of selenium or selenium alloys involves several disadvantages in that the film formation essentially requires the complicated step of vacuum evaporation and in that the vacuum evaporated film formed has poor flexibility.
It is also known that zinc oxide or cadmium sulfide is also usable as an electrophotographic photosensitive material by dispersing such in a binder resin. However, the dispersed photosensitive resin layer does not withstand repeated use because of the low mechanical strength of the layer. Especially, the spectral sensitivity of zinc oxide is unsatisfactory, so that use of sensitizing dyes for spectral sensitization is essential to widen the sensitivity range of zinc oxide toward the long wavelength range.
In addition to inorganic photosensitive materials such as selenium, zinc oxide, cadmium sulfide, etc., organic photosensitive materials are widely known, of which polyvinylcarbazole and derivatives thereof are typical. Of these, polyvinylcarbazole per se has been intensively studied as is known from the abundant literature available. Although polyvinylcarbazole has excellent transparency, film-forming ability and flexibility, polyvinylcarbazole does not exhibit any sensitivity in the visible light range by itself. Accordingly, a number of methods have been proposed for sensitizing polyvinylcarbazole e.g., as disclosed in U.S. Pat. No. 3,484,237, U.S. Pat. No. 3,655,378, etc. Typical sensitization methods include spectral sensitization using dyes such as triphenylmethane dyes, pyrylium salt dyes, etc., and chemical sensitization using electron-accepting compounds of which tetranitrofluorenone is typical. In spectral sensitization, polyvinylcarbazole sensitized with dyes has a spectral sensitivity range expanded over the visible light range, but has unsatisfactory sensitivity for use as an electrophotographic photosensitive material. In addition, dye-sensitized polyvinylcarbazole has a high tendency toward light fatigue and can not thus withstand repeated use. While, in chemical sensitization, some combinations of polyvinylcarbazole and electron-accepting compounds have been found to exhibit a satisfactory sensitivity for use as an electrophotographic photosensitive material and are now employed in practice. However, photosensitive materials of the type just mentioned have several problems in terms of mechanical strength, life time, etc.
In recent years, pigment-dispersed photosensitive materials wherein photoconductive pigments are dispersed in a binder have been extensively studied and a number of reports concerning such photosensitive materials have been made as described in, for example, U.S. Pat. No. 3,887,366. However, photosensitive materials which have excellent electrical characteristics and satisfactory sensitivity suitable for electrophotographic purposes have not yet been obtained. For example, a report that phthalocyanine has excellent characteristics as a dispersion-type photosensitive material has been made. However, the spectral sensitivity of phthalocyanine is inclined towards the long wavelength region, resulting in poor fidelity for red color. Further, Japanese Patent Application (OPI) No. 99142/1975, for example, discloses that a photosensitive material obtained by dispersing an organic photoconductive compound and a spectral sensitizing agent for the compound in a binder has excellent electrical characteristics. However, the mechanical strength and life time of such a photosensitive material are unsatisfactory and must be improved.